Prevent duplicate username in c# window form

Question

con.Open();
            SqlCommand cmd = con.CreateCommand();
            cmd.CommandType = CommandType.Text;
            cmd.CommandText = "insert into Login values('" + textBoxUsername.Text + "', '" + textBoxRetype.Text + "')";

            if (cmd.ExecuteScalar() != null)
            {
            }
            else
            {
                MessageBox.Show("Username exist");
                con.Close();
            }

            cmd.ExecuteNonQuery();
            cmd.CommandText = "insert into Employee values('" + textBoxName.Text + "','" + textBoxEmail.Text + "','" + textBoxAddress.Text + "','" + textBoxMobile.Text + "')";
            cmd.ExecuteNonQuery();
            con.Close();
            display_data();
            MessageBox.Show("Account Registered!");

Im usimg c# windows form in Visual Studio 2017. From the above coding, i have used cmd.ExecuteScalar to prevent users from using the duplicate usernames but  its not working. When i clicked on the register button after keying in all the name,email and etc.... , the message box will keeo showing "Username exisit" even if its not duplicate usernames.